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The available dive time for a SCUBA diver is a function of his compressed air supply. A freshly pressured tank usually has elevated temperature due to the heat generated by the compressor, if one is used, or due to the heat picked up as gas expands out of a high pressure storage tank and compresses into the SCUBA tank. As a warm, filled tank equilibrates with cooler ambient air or water, the pressure drops. This amounts to an ullage in the freshly filled tank. This ullage represents a concomitant loss in available dive time compared to that of the same tank filled to capacity with temperature-equilibrated air.
Divers use their compressed air tank pressure to predict how long they can stay down. Normally, a diver desires a maximum dive timexe2x80x94a maximum pressure at the start of a program of diving. Also, at the time a tank is filled, the diver would like to know how much air he actually will have after the tank temperature finally equilibrates A tank filled to capacity with warm air will have less useful air than might be indicated by full pressure before the tank cools off.
Any prediction of ullage for a freshly filled tank must be based on a temperature measurement of the freshly filled tank. Santacaterina (U.S. Pat. No. 4,779,995) has invented a system of color temperature strips. His patent lists sixteen other related patents. Paron (U.S. Pat. No. 5,738,442) has invented a temperature indicator for a wine bottle based on art similar to that of Santacaterina. Neither pressure nor ullage is related to these patents. Yet, these patents use the kind of temperature sensitive material that might be used in the present concept. The material changes color at a particular temperature depending upon the design of its makeup.
Teletemp company of Fullerton, California makes one type of temperature indicator which can be used in the present invention. Liquid crystals are formulated for a specific temperature response. In the response reflected light is modified to allow a message such as a specific pressure to be seen. Unless the temperature is within the designed range of the liquid crystal material, the indication is black. The adhesive-backed liquid crystals have been used to indicate body temperature, as refrigerator thermometers and as hot warning labels to name a few.
Gas storage tank cooling after filling and the prediction of the future air supply from a cooling tank has been a problem without solution for divers in spite of the great expansion of SCUBA sport over the past forty years. The diver has no real use for the tank temperature. What he really needs is a method to provide an indication of what the supply of air will be from the warm tank supply after the tank has cooled. The diver knows his tank and it""s capacity in terms of minutes of divingxe2x80x94if it is completely filled at the start of a dive with temperature-equilibrated air. The method of this invention tells the change in pressure to expect when the tank temperature equilibrates. With this invention indicators on the tank surface that are actuated by the tank wall temperature do this. Change in color due to a particular temperature allows a particular ullage number to be seen. The ullage number is pre-calculated based on the gas law of temperature effect on pressure at constant volume. Each ullage number is based on one hypothetical final gas temperature or a variety of segments can be used, each not only based on a given temperature immediately after filling, but also each based on an expected equilibrated final temperature.